The invention relates generally to detector arrays and, in particular, to the use of redundant two-dimensional (2-D) detector arrays in sensor assemblies to improve performance and operability over that obtainable from a single detector.
A simplified sensor assembly design is shown in FIG. 1. In practice, the sensor assembly would have several optical elements, the net result of which is to focus the light onto the detector array and form an image. While a simplified refractive sensor assembly design is shown in FIG. 1, mirrors could also be used for image formation.
In the specific case of infrared (IR) sensing, the typical detectors used are Focal Plane Arrays (FPAs). The FPA is the major design driver in the infrared sensor. The number of pixels (individual sensing elements) in the FPA ultimately dictates the spatial resolution of the sensor; therefore, a sensor designer will generally seek to maximize the pixel count when selecting the FPA. Unfortunately, as pixel count increases, the likelihood of FPA manufacturing defects also increases. The measure of quality is “operability,” defined herein as the percentage of the pixels that are functional and produce an electrical response to incident light within a specified operating range of wavelength, photon fluence, and temperature. A decrease in manufacturing quality is experienced in two primary ways: individual pixels will be non-functional and have no output; or individual pixels will have a very low or very high output electrical response to incident energy. An example “operability” specification might read, “The FPA will have 0.90 operability when 90% of all pixels have quantum efficiency (QE) greater than 25% of the median QE value.”
State of the Art IR FPAs have over 4-million pixels in array sizes of 2048×2048 (commonly abbreviated as 2k×2k). To manufacture high-quality FPAs, industry typically produces large batches that are screened to select the few FPAs with operability greater than 90%. To illustrate the manufacturing difficulty, to date few 2k×2k FPAs have been produced with greater than 95% operability and no 2k×2k infrared FPA has been built with 100% operability.
FPA operability over 90% is acceptable for many applications. There are a few critical applications, however, where perfect or near-perfect operability is required. An example might be the use of IR FPAs for missile detection from spacecraft. In this application, the need for high operability is based on the size of the target with regard to pixel size. Because such a target may be contained within a single pixel, dead (non-working) pixels corresponding to blind spots in the field-of-view would result in a failure to detect the missile. The blind-spot problem is exasperated when dead pixels are contiguous causing clumps of outages, which is a common manifestation of the manufacturing defects in IR FPAs.
For applications that can tolerate the use of FPAs with typical 90% to 95% pixel operability, a sensor design using even lower quality FPAs could be advantageous. Such a design would reduce the long lead times needed for the batch screening process (typically 12 months) and reduce cost since fewer FPAs need to be manufactured in the batch process. The system reliability of such a design would also be improved, allowing for redundant strings or graceful degradation.